Many ion sources, which had been designed for space propulsion applications, have been applied to material processing. Gridded ion sources, which make use of electrostatic ion acceleration optics (grids) to accelerate ions from a low pressure gas discharge, have been routinely used for ion sputtering, ion implantation, surface modification, dual ion-beam sputter deposition and ion-beam-assisted-deposition techniques to form thin films and coatings. Numerous gridded ion sources have been devised, each with its own particular means of producing or heating electrons to form a low pressure gas discharge and means of producing an ion beam by electrostatic acceleration optics. Some examples of means for electron production and heating include the use of hot filaments, high-field electron emission, RF capacitive heating, RF inductive heating, and microwave electron cyclotron resonant RF heating. Once a gas discharge is formed, the ion beam is extracted and accelerated to select energies by an electrostatic grid or system of electrostatic grids. Downstream of the ion acceleration optics, a secondary electron source is often used to neutralize the space-charge of the ion beam in order to reduce beam divergence and surface charging of targets or workpieces.
In some processes, high ion beam current densities (&gt;1 mA/cm.sup.2) are desired for rapid treatment or deposition rates. To achieve rapid processing rates, particularly over large areas, very high total beam currents and current densities with a broad area ion beam are required. Many advances have been made in forming relatively high charge-particle densities within the ion source (10.sup.11 to 10.sup.12 /cm.sup.3) in order to support a high beam current flux density from an ion acceleration optic means. At charged-particle densities greater than 10.sup.11 /cm.sup.3, however, there are certain limits of the ion acceleration grids that restrict the ion current density and total ion beam current which may be continuously extracted from the ion source and accelerated. Since the electrostatic lens action of an ion acceleration optic system virtually excludes electrons, there are inherent space-charge limits to the ion beam current density extracted by grid optics. Also, the total ion beam current throughput is restricted by thermal and mechanical limitations of ion acceleration grids. In sputtering and depositing environments, grids can become coated with either conductive or insulating coatings. Gridded ion beam sources used in such applications are prone to fail either through short circuits or dielectric shielding as a result of the spurious coatings. Grids will erode in time due to direct ion sputtering. As such, conventional ion beam grids deteriorate over time, introduce contaminants, and add considerable maintenance and reliability costs when used in many material processing applications.
To overcome the limitations imposed by ion acceleration grids, several workers have developed gridless DC ion sources. In these DC or pulsed-DC devices, ions are accelerated from a region of ion production through an electric field, E, established within the bulk of the discharge near the anode of the apparatus. The electric field is brought about by a static or quasi-static magnetic field, B, imposed on the discharge in the vicinity of an anode wherein the electron drift motion from cathode to anode is impeded by the magnetic field. Electrons formed at the cathode ionize feed gases as they drift toward the anode through the magnetic field via collisional and anomalous diffusion. The restricted mobility of electrons across the magnetic flux forms a space charge near the anode and a relatively strong electric field that is substantially orthogonal to the imposed magnetic field and anode surface. Ions generated within anode discharge region are accelerated away from the anode. Since the anode discharge and ion acceleration regions do not exclude electrons, ion beam current densities are not restricted by space-charge limitations that are inherent in electrostatic acceleration optics. The electrons formed at the cathode and within the discharge also serve to electrically neutralize the ion beam as it propagates away from the anode's ion acceleration region. At pressures above 10.sup.-4 Torr, ionization away from the anode discharge region and charge-exchange processes within the ion beam can form a diffusive background discharge making the output characteristics of the source appear as both an electrically neutralized, energetic ion beam and a diffusive background plasma. The combined output of both a self-neutralized ion beam and diffusive plasma is sometimes referred to as a "plasma beam".
Another characteristic feature of this type of ion source is an E.times.B drift current motion of electrons in the acceleration region. Electrons, which spiral about the lines of magnetic field, experience an E.times.B or Hall-effect force and collectively drift in a direction perpendicular to both electric and magnetic fields. This is referred to as a Hall-effect drift current. In order to avoid Hall potentials which may form along this electron drift path, these ion sources have anode discharge regions or channels that allow Hall-effect drift current to flow along a continuous and closed path. Workers have referred to these types of ion sources by many names: "Magneto-Plasma-Dynamic Arc Thrusters", "Hall-Accelerators", "Closed-Drift Thrusters", and "Hall-Current Ion Sources". For the purpose of discussion and teaching, we refer to these types of devices, in general, as "Hall-Current ion sources".
The following references illustrate the prior art with regard to Hall-Current ion sources.
The body of work on Hall-Current ion sources that have been developed for space propulsion applications is summarized by H. R. Kaufman in "Technology of Closed-Drift Thrusters" AIAA Journal Vol. 3, pages 78-87 (1983) and in the references cited therein.
Burkhart, U.S. Pat. No. 3,735,591 discloses and claims a Hall-Current ion source termed "Magneto-Plasma-Dynamic Arc Thrusters" for space propulsion applications.
Cuomo et al., U.S. Pat. No. 4,541,890 disclose and claim a high current density, low ion energy Hall-Current ion source for use in integrated circuit manufacturing processes.
Kaufman, U.S. Pat. No. 4,862,032, discloses and claims a so-called "End-Hall" ion source for the production of high current density, low energy ion beams.
Okada et al., Japanese Journal Applied Physics, Vol. 31, pages 1845-1854 (1992), describe a high energy Hall-Current ion source used for ion implantation and deposition of diamond-like carbon (DLC) coatings.
The articles by Chuzhko, et al. in Diamond and Related Materials, Vol. 1, pages 332-333 (1992), and Fedoseev, et al. in Diamond and Related Materials Vol. 4, pages 314-317 (1995) describe a Hall-Current ion source used in deposition of DLC coatings.
Hall-Current ion sources have been used to generate chemically inert and reactive ion beams for space propulsion and materials processing applications. While several designs have been developed, Hall-Current ion sources may be classified into three basic configurations as illustrated in FIGS. 1A through 1C. FIGS. 1A-1C distinguish between the non-magnetic stainless steel (SS) of the anode, the magnetic stainless steel of the pole pieces and insulative material, which are often used in these ion sources.
FIG. 1A shows "Extended Channel" Hall-Current ion source 10 with extended anode discharge region or acceleration channel 14. Ion source 10 comprises several parts: circular, cylindrical channel or anode discharge region 14 consisting of an insulative material and having opening 16 at first end 17 and at least one flat ring anode 18 consisting of a non-magnetic stainless steel located within and adjacent second end 20 of channel 14; gas feed line 22 communicating with anode discharge region 14 behind anode 18 at second end 20; magnetic circuit having pole pieces 26 consisting of magnetic stainless steel and forming a radially directed magnetic field, B; one or more electromagnets 30 (or permanent magnets 30); cathode 34; and discharge power supply 38 electrically connected between cathode 34 and anode 18. Extended channel 14 has an aspect ratio of channel length, L, over channel width, W, greater than 1 (L/W&gt;1).
FIG. 1B shows "Space-charge Sheath" Hall-Current ion source 40 having much shorter cylindrical channel 44 than channel 14, typically channel 44 has L/W&lt;1. In addition, ion source 40 has ring anode 48 having groove or channel 50. This type of Hall-Current ion source with a relatively shorter anode discharge region produces an ion beam with mean energies that are typically lower than that of the extended channel type, but has the advantage of greatly reduced discharge losses to the walls which bound the anode discharge region. In the ion source of FIG. 1B, the electron current between cathode and anode principally makes contact to the inner and outer tips of the grooved ring anode 48 which extend into and cross the magnetic field lines.
FIG. 1C shows yet a third type of Hall-Current ion source of the prior art, "End-Hall" ion source 60. In End-Hall ion source 60, magnetic pole pieces 26 are arranged to form an end-divergent magnetic field that is directed along the axis of the ion source and is directed through opening 16 and to outer pole 28. Conical, annular 64 defines the anode discharge region 68.
Hall-Current ion sources must meet several criterion in order to be suitable for production at high deposition rates. It should be noted that many of these criterion discussed in some detail below apply to both ion beam deposition and non-deposition processes.
First, high deposition rates generally require high discharge power levels. Typically 50 to 70 percent of the electrical power delivered to a Hall-Current ion source is directly or indirectly lost to heating of the ion source components. If the source is passively cooled by radiative thermal emission in vacuum, thermally sensitive workpieces, such as plastics may be damaged by thermal flux from the hot ion source assembly. Thus, in order to facilitate high deposition rates, it would be desirable to cool the Hall-Current ion source by means other than radiative thermal emission.
Second, the ion source must operate robustly in production. The ion source must ignite reliably and easily and have the broadest possible operating range in power and pressure. The source should not operate in any manner that would degrade its own internal components or output efficiency. The output properties of the plasma beam should remain consistent over time and should be substantially uniform or at least symmetric with respect to the scale and symmetry of the apparatus. Moreover, all of these attributes should consistently hold for the ion source throughout prolonged periods, i.e., &gt;20 hours of continuous coating operation. For applications where conductive coatings are generated, (coatings with bulk resistance substantially &lt;10.sup.2 Ohms-cm), spurious deposition over non-conducting surfaces on the ion source must not lead to short circuits between electrically active components. Conversely, in applications where non-conducting coatings are produced, (coatings with bulk resistance substantially &gt;10.sup.2 Ohms-cm), electrically critical surfaces on the ion source must not be entirely coated with insulating deposits.
Third, the Hall-Current ion source should be scalable such that large surface areas can be processed with minimal workpiece manipulation within the plasma beam in order to achieve a desired coating uniformity.
The use of an End-Hall ion source for direct ion beam deposition of abrasion resistant coatings is described by Knapp et al., U.S. Pat. No. 5,508,368. They disclose a process which deposits highly abrasion resistant coatings with thicknesses of 1 to 10 .mu.m over various substrates including plastic materials such as acrylic and polycarbonate. The coatings were produced by injecting precursor gases and vapors directly into the plasma beam downstream of the ion source anode discharge region. In their work, total volumetric coating rates were in the range of about 0.01 to 0.1 cm.sup.3 /min. However, when attempting to apply End-Hall and other Hall-Current ion sources of the prior art to the process disclosed by Knapp et al. at higher deposition rates (0.1 to 1 cm.sup.3 /min.), many technical problems arise concerning the performance of the ion source apparatus.
For example, several complications arose when attempting to adapt either of two commercial versions of the End-Hall ion source (the Mark II End Hall Ion Source and the Mark III End Hall Ion Source, manufactured by Commonwealth Scientific Corporation, Alexandria, Va.) into a commercial production setting for coating plastics at very high deposition rates. These End-Hall ion sources were radiation cooled and as such their power ranges were very limited when coating thermally sensitive plastics for prolonged periods due to the high thermal power flux emitted from hot ion source components. Furthermore, the ion sources were prone to physically sputter metal from the gas distributor plate located near the base of the anode assembly. Typically the potential of this electrically floating plate is many tens of volts lower than that of the anode and some of the ions produced in the anode discharge region are accelerated back toward the gas distribution plate. This ion bombardment keeps the gas distributor plate relatively free of non-conductive deposits, but heats the electrically floating plate and sputters metal contaminants into the plasma beam. Such metal contaminants were found to cause poor processing performance, e.g., poor film adhesion, optical defects and the like, over the treated workpiece. Also, the anode assemblies in the commercial End-Hall ion sources would frequently arc to both grounded and floating metal components within the ion source body. These random transient arcs sputtered metal onto electrical insulators within the assembly causing electrical short circuits and would often extinguish the discharge during a coating operation, with no simple means to immediately re-ignite the discharge. Moreover, these arc events became more pronounced and frequent when using hard-to-ionize feed gases, when operating at higher power levels, and when depositing non-conductive coatings.
Given these problems, the End-Hall ion sources described by Kaufman in U.S. Pat. No. 4,862,032 and commercially manufactured by Commonwealth Scientific Corporation were unsuitable for production of coatings deposited by means disclosed in the Knapp et al. U.S. Pat. No. 5,508,368 when attempting to operate these ion sources at very high deposition rates. Furthermore, there is no method or teaching disclosed in the Kaufman '032 patent, nor means obvious to one of ordinary skill in the art that would lead to solutions to these shortcomings.
Other patents and reports in the prior art describe the use of Hall-Current ion sources for prolonged operation in either inert or reactive gases and for direct deposition of material from the ion source plasma beam. Yet none describe or teach how to overcome problems associated with depositing coatings, particularly those problems associated with high rate deposition of non-conductive coatings over thermally sensitive materials.
The "Magneto-Plasma-Dynamic Arc Thruster" described by Burkhart, U.S. Pat. No. 3,735,591 and referred to above is radiation cooled and operates at power levels that are too low or at temperatures that are too high to support very high deposition rates on thermally sensitive materials. Also, this device uses a cathode located directly within the plasma beam, an undesirable condition in that the cathode can electrostatically perturb the plasma beam and become a source of sputter contamination. In this Hall-Current ion source, gas is injected into the source by means of at least one tube through the hollow, cylindrical anode wall. As will become evident in other prior art examples, prolonged operation of this ion source during the deposition of non-conductive coatings will concentrate the discharge current at the gas feed opening, producing a non-uniform plasma beam and damaging, i.e., by melting or vaporizing, the anode wall about the gas inlet when operating at high discharge currents.
Similar problems are observed in all prior art Hall-Current ion sources developed for space propulsion applications. All of these Hall-Current ion sources emphasize low power operation, lightweight components, high specific impulse and thrust efficiency, and long-life operation when using chemically inert propellant fuels such as argon or xenon, or in some cases easily ionizable metals such as cesium. A common feature of space propulsion Hall-Current ion sources is that they are cooled by radiative thermal emission. In continuous operation they can be operated only at relatively low power, and this limitation becomes more restrictive as they are made more compact and lightweight, as desired for space propulsion applications. As such, the prolific literature in this area provides no teaching on means or methods related to the use of Hall-Current ion sources for operation at high power and in chemically active or depositing environments.
In the preferred embodiment described in Cuomo et al., U.S. Pat. No. 4,541,890, of a radiation cooled Hall-Current ion source, working gases are introduced into the anode discharge chamber from behind and adjacent the anode by a separate annular manifold that is also electrically isolated from the anode. As will become evident in later prior art examples, this means of gas distribution may be made to operate with inert, non-depositing gases. However, in the case of depositing gases, this same Hall-Current ion source is prone to failure. Non-conductive deposits can readily form on those regions of the anode that are exposed to the depositing environment, after which the electrically active anode area contracts to surfaces behind the anode where line-of-sight deposition is low or negligible. Channeling of high energy electrons into these areas can drive intense discharge activity behind or alongside the anode assembly, rather than within the anode discharge region as desired for efficient operation. Moreover, the intense discharge activity between the anode and non-anode surfaces, such as the gas distribution manifold, can lead to ion sputtering and/or overheating of either grounded or floating metal or insulating non-anode surfaces. Depending on the assembly, sputtered metal can form short circuits across insulating hardware and inject metal contaminants into the process. It should be noted that Cuomo et al. do not discuss or teach any means by which to address the disabling problems that would be encountered in their ion source when applied to high rate deposition processes.
In a recent work by Feedoseev, et al., described in the article referred to above, a so called "Hall Accelerator" ion source with a water-cooled anode was used to produce DLC coatings from a mixture of hydrogen, argon, and methane feed gases. In their Hall Accelerator design, gases are delivered uniformly into the anode discharge region through a 0.02 cm wide annular slot in the base of a V-shaped, annular anode. While it has been demonstrated that this ion source can be used to deposit DLC coatings from hydrocarbon feed gases, it was observed that this same ion source failed when attempts were made to integrate it into the process disclosed by Knapp, et al. in the above reference. Non-conductive deposits formed on the Hall Accelerator anode, which reduced the electrically active surface area and increased the anode potential. Eventually the potential on the anode was sufficiently high for the gas behind the anode to break down, and arcs occurred to conductive surfaces within or behind the ion source. The presence of non-conducting coatings on the anode also made it very difficult to re-ignite the ion source. It is known that during DLC deposition, deposits on hot ion source surfaces can be graphitic in composition and thus electrically conductive. Presumably, this occurred on areas of the Hall Accelerator anode that were insufficiently cooled, such as at the tips of the V-shaped anode where much of the discharge electron contact current is intended to occur. In the process described by Knapp, et al., however, deposits on both hot and cold regions on the anode are non-conductive, and thus their presence disabled the Hall Accelerator ion source.
A Hall-Current ion source with an extended channel was used by Okada et al. in the work described in the article referred to above to deposit DLC films from a combination of argon and various hydrocarbon gases. The relatively bulky and sophisticated device uses many electromagnets to form a magnetic field within its extended acceleration channel. There is no direct, active cooling of the anode assembly and the ion source does not use a self-sustained cathode. There have been no disclosed reports on the use of this apparatus in environments where non-conductive deposits form on the hot anode. Also, there are no disclosed embodiments within this extended channel Hall-Current ion source that possess unique advantages over any other Hall-Current ion source of the prior art with regard to common problems encountered in direct deposition of non-conductive coatings.
The following general technical problems confront the application of prior art Hall-Current ion sources to processes where high rate deposition or high power operation are required.
The radiation-cooled Hall-Current ion sources of the prior art are not suitable for rapid treatment and coating of thermally sensitive materials. During high power operation, the radiative thermal energy from the hot ion source will heat and potentially damage thermally sensitive workpieces. Thus, it is important to extract thermal energy from the ion source apparatus by a means other than radiative thermal emission.
When depositing non-conductive coatings with Hall-Current ion sources, coatings will cover those areas of the clean anode surface that are exposed to the anode discharge region. This decreases the active anode area and eventually causes the ion source to fail in a number of ways. In ion sources of the prior art where gas is delivered around the anode, the active anode surface contracts to areas to the side and behind the anode, and as a result, power delivered to the discharge tends to be diverted into wall recombination losses about the perimeter and behind the anode, rather than into volume ionization and ion acceleration within the anode discharge region. In ion sources of the prior art where the gas is injected through the anode by one or more discrete holes, the active anode area contracts to a high current density region within close proximity about the discrete gas injection hole(s). In the case of one hole, the discharge in the acceleration channel and the resulting plasma ion beam profile becomes non-uniform or asymmetric. Moreover, the intense electron current to the small conductive surface area can locally melt and evaporate the anode metal when the source is operated at high current levels. In the case of an array of holes or a thin continuous slot in the anode that is directly open to the anode discharge region, the active anode surface can diminish to such a degree, particularly during high-rate deposition conditions, that the ion source will become unstable and rise outside its anode voltage operating range. Eventually the ion source discharge current will become extinguished or fail to flow solely between the cathode and anode discharge region.
When depositing conductive coatings with Hall-Current ion sources of the prior art, spurious deposition on the non-conductive surfaces can degrade the isolation between electrically active components. Early on this will lead to unknown power losses that can reduce deposition rates. Eventually the anode-to-cathode potential drop will decrease to the point where the plasma can no longer be sustained.
Many of the Hall-Current ion sources in the prior art also exhibit unstable performance, often operating with difficulty to ionize gases, as a result of either instabilities in the discharge or "sparks" or "arcs" between the anode discharge region and metal boundaries near this region. These events can diminish or divert the discharge current within the anode discharge region and disrupt the discharge properties, ie., charged-particle densities and plasma potential fields, to such a degree that the discharge will be extinguished. It is desirable to have a Hall-Current ion source that does not exhibit such instabilities or arcing events or that is at least insensitive to their occurrence.
Many of the Hall-Current ion sources in the prior art have metal components that bound the anode discharge region. Ion bombardment can sputter metal from these surfaces even under conditions of non-conductive coating deposition where the concomitant ion sputtering and heating of such surfaces compete with deposition. It is desirable to eliminate or minimize sputtering from all metal components which can contaminate the coating process or which can lead potentially to electrical short circuiting of the anode.
Nearly all Hall-Current ion sources of the prior art make use of either permanent magnets or electromagnets driven with an independent power supply to form a static magnetic field. As such, these Hall-Current ion sources do not always ignite easily and reliably over their desired range of operation. Because the anode voltage threshold and feed gas levels required to breakdown the working gases are greater than those required for steady-state operation. Thus, the ignition process of Hall-Current ion sources of the prior art have an inherent hysteresis. Workers must alter the magnetic field strength, induce a high-voltage wave form, or alter gas flows dynamically in order to ignite the discharge and then re-adjust these properties to desired set points. A less complicated ignition procedure is desired to easily ignite the discharge and operate the ion source over its broadest possible steady-state range. Rapid and easy ignition is particularly desirable for rapid-rate deposition of very thin coatings (50 to 100 Angstroms) which may require only a few seconds of ion source operation.
It is difficult to scale conventional Hall-Current ion sources to process large areas without significantly increasing their gas feed requirements for stable operation. All Hall-Current ion sources of the prior art make use of a closed-path anode discharge region in order to avoid the formation of asymmetric Hall-potentials along the E.times.B drift path of the electrons, which in turn leads to asymmetric plasma beam properties. In some processes, it would be highly advantageous to form a linear plasma beam that is distributed over a large surface such as a moving sheet, fixture, flat panel or web. Such a linear Hall-Current ion source built with a conventional closed-path must have a closed-drift anode discharge region that is at least twice the length of the linear ion source. It has been observed that the gas load requirements of a Hall-Current ion source tends to scale with the circumference or length of the closed-path of the anode. Thus, a large linear Hall-Current ion source configured with a conventional closed-path would require substantially high gas feed levels and high vacuum pumping speed for stable operation. Yet it generally is not desirable to scale gas and pumping requirements in an effort to merely re-distribute the geometry of the ion source's output. It is more desirable to have a Hall-Current ion source that is not restricted by the closed-path convention of the prior art but that does not exhibit asymmetric Hall-potentials and similar asymmetries in its plasma beam properties.
In order to geometrically distribute or scale the output of the plasma beam from Hall-Current ion sources of the prior art, multiple ion sources with multiple anodes and cathode power supplies and, in many cases electromagnet power supplies are required. It would be desirable to distribute the power from one or more power supplies to several anodes or anode discharge regions, such as an array of anodes, by distributing the discharge current between two or more anodes and then combining the currents to a single common cathode. Such a method would minimize the number of power supplies and ion source components necessary to form a plasma beam for processing large areas. Also, such an approach requires a means by which to balance the ratio of discharge current and power delivered to each anode in the ensemble. Aside from ganging multiple Hall-Current ion sources together, there is no teaching in the prior art on means by which to electrically combine and control such an ion source system.
Set forth below is a summary of the shortcomings of the Hall-Current ion sources of the prior art.
(1) Radiative thermal emission that is detrimental to thermally sensitive workpieces. PA1 (2) Extensive coating of the anode surface with non-conductive coatings inherent in the process that leads to: PA1 (3) Unreliable operation in environments where conductive coatings are deposited. PA1 (4) Frequent arcing between the anode and non-anode metal components and sensitivity to transient arcs and instabilities that lead to loss of the discharge current or its redirection outside the anode discharge region. PA1 (5) Metal sputtering from metal components of the ion source within the anode discharge region or within the plasma beam. PA1 (6) No simple, rapid and electrically passive means by which to ignite the ion source discharge or re-establish the ion source discharge current in the event of an inadvertent, transient loss of the discharge current. PA1 (7) No means by which to scale a Hall-Current ion source with a non-closed anode path and means to avoid asymmetric Hall-potentials in the anode discharge region and in the plasma beam. PA1 (8) No simple means by which to connect multiple ion source systems or anodes in parallel with a common cathode and anode power supply so as to distribute and control the discharge power, currents, and plasma beam properties over large processing areas. PA1 (a) an anode discharge region for the formation and acceleration of a plasma beam; PA1 (b) an insulatively sealed anode to prevent plasma from forming behind the anode; PA1 (c) a non-radiative cooling means for cooling the anode; PA1 (d) a self-sustaining cathode, i.e., a cathode having an independent power supply; PA1 (e) an electromagnetic means that operates at least partially on either the discharge current from the anode to the self-sustaining cathode or current from an independent, periodically reversing or alternating current; and PA1 (f) a gap within the anode to introduce plasma maintenance gas or working gas. PA1 (a) an anode discharge region for the formation and acceleration of a plasma beam with the anode bounded by one or more continuous thin gaps so disposed to prevent deposition in the gaps which in turn prevents the formation of conductive paths between the anode and other parts of the ion source adjacent to the anode; PA1 (b) an insulatively sealed anode to prevent plasma from forming behind the anode; PA1 (c) a non-radiative cooling means for cooling the anode; PA1 (d) a self-sustaining cathode; PA1 (e) an electromagnetic means that operates at least partially on either the discharge current from the anode to the self-sustaining cathode or current from an independent, periodically reversing or alternating current; PA1 (f) a gap within the anode to introduce plasma maintenance gas or working gas; and PA1 (g) distribution means for introducing and distributing depositing gases directly into the plasma beam. PA1 (a) an anode discharge region for the formation and acceleration of a plasma beam; PA1 (b) an insulatively sealed anode to prevent plasma from forming behind the anode; PA1 (c) a non-radiative cooling means for cooling the anode; PA1 (d) a self-sustaining cathode; PA1 (e) an electromagnetic means that operates at least partially on either the discharge current or current from an independent, periodically reversing or alternating current; PA1 (f) a gap within the anode to introduce plasma maintenance gas which provides an anode surface area within gap that remains substantially free of non-conductive deposits; and PA1 (g) distribution means for introducing and distributing depositing gases directly into the plasma beam.
(a) non-uniform or asymmetric discharge formation about the anode discharge region where ions are principally produced and accelerated; PA2 (b) contraction of the electrically active anode surface to areas along side and behind the anode; PA2 (c) localized, high discharge current densities at the anode that lead to damage of the anode surface; and PA2 (d) loss or disruption of the discharge current between the anode discharge region and cathode.